Semiconductor laser device and a method of producing same

ABSTRACT

A semiconductor laser device of the buried heterostructure type in which leakage current is substantially reduced. A mesa portion carries the active lasing portion of the laser, and current blocking layers are grown at either side of the mesa portion. One of the current blocking layers has its conductivity type inverted to the opposite type to eliminate a current leakage path, thereby to provide a high efficiency low leakage current semiconductor laser. Conductivity inversion is accomplished by adjusting the impurity concentration levels in the layers on either side of the mesa portion, and controllably diffusing impurities from one layer to another until conductivity inversion is accomplished in a thin tip portion of one of the layers.

FIELD OF THE INVENTION

This invention relates to an improved semiconductor laser device and amethod for producing such a device.

BACKGROUND ART

Semiconductor laser devices have been produced but they have had variousdrawbacks, such as difficulty of production or, when produced, excessiveleakage current resulting from unwanted contact between certainsemiconductor layers in the device.

FIG. 6 shows a cross-sectional illustration of a typical prior artsemiconductor laser device described in Japanese Laid-open PatentPublication No. 61-204994. That structure is based on a p typesemiconductor substrate 1 having an active layer 2 grown on thesubstrate, and an n cladding layer 3 grown on the active layer. Afterthe two layers are grown on the substrate during a first liquid phaseepitaxial growth process, a pair of channels are etched to produce amesa structure generally designated at 1' in which a central stripecarrying the active layer and n cladding layer is disposed above thesubstrate. In order to confine the current within the active layer 2,additional p-n layers are grown on either side of the mesa portion. Inthe illustrated embodiment, a p type embedded layer 4 is grown over thesubstrate (as well as over the portions of the active layer 2 and ncladding layer 3 near the edges of the device). Following growth of thep type embedded layer 4, additional layers are grown including an n typecurrent blocking layer 5 and a p type current blocking layer 6. Finally,an n type cladding layer 7 is grown over the entire top surfaceincluding the cladding layer 3 and the p type current blocking layer 6to form a substantially flat portion for receipt of an electrode (notillustrated).

In operation, when a voltage is applied between the p type substrate 1and the n type cladding layer 7, the holes and electrons which are thecarriers of the respective layers are injected into the active layer 2.When the injection current reaches a predetermined (preferably fairlylow) level, laser oscillation occurs and light is emitted. The injectedcarriers are concentrated in the active layer 2, in part by theautomatically reversed biased p-n junction comprised of the currentblocking layers 5, 6. By virtue of the barrier created by the reversebiased p-n junction 5, 6, current is largely confined within the activeregion, and the semiconductor laser device should operate at a highefficiency and with low leakage or idle current.

In practice, however, such a laser device does not attain the expectedhigh efficiency because of leakage currents which are created by virtueof the juxtaposition of certain layers in the device. More particularly,in construction of such a device there is contact, and therefore anelectrical connection, usually created between the n type currentblocking layer 5 and either the n type cladding layer 3 or the n typecladding layer 7, and that connection results in a leakage currentthrough the device. The leakage or idle current which flows in thedevice is illustrated by the arrows shown in FIG. 6, and is relativelyhigh considering that the resistivity of the n type layer is less thanthat of the p type layer by about an order of magnitude. Furthermore,since the area of the p-n junction produced between the p type embeddedlayer 4 and the n type current blocking layer 5 is large, the totalleakage current from the n blocking layer to one of the n claddinglayers will be quite high. Indeed, it has been found, that when thesemiconductor laser is produced by liquid phase epitaxy, the n typecurrent blocking layer 5 and one or the other of the n cladding layers3, 7 will be connected in almost all cases. Although the width of thisconnection is generally small, such as 0.1 μm, a large leakage currentwill flow for the reasons described above.

SUMMARY OF THE INVENTION

In view of the foregoing, it is a primary aim of the present inventionto provide an improved semiconductor laser device which can be reliablymanufactured, and which, in operation, will reduce leakage current whichheretofore had been present between one of the current blocking layersand cladding layers.

More particularly, it is an object of the present invention to provide adefinite separation between the n type current blocking layer and the ntype cladding layers in such a semiconductor laser device. It is afurther object to provide such a definite separation while not undulycomplicating the manufacture of the semiconductor laser device.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when taken inconjunction with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a semiconductor laser deviceembodying the preset invention;

FIGS. 2a to 2c are diagrams representing the manufacturing process forproducing a semiconductor laser according to the present invention;

FIG. 3 is a cross-sectional view showing a semiconductor laser device inaccordance with a further embodiment of the present invention;

FIGS. 4 and 5 are graphs exemplifying the growth process for particularlayers of the semiconductor laser according to the present invention;and

FIG. 6 is a cross-sectional view showing a prior art semiconductor laserdevice.

While the invention will be described in connection with certainpreferred embodiments, it will be apparent that there is no intent tolimit it to the described embodiments. On the contrary, the intent is toencompass all alternatives, modifications and equivalents includedwithin the spirit and scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIG. 1, there is shown one embodiment of a semiconductorlaser device constructed in accordance with the present invention. Inthe illustrated embodiment, a p type semiconductor substrate serves asthe foundation for the semiconductor. A mesa portion 1' locatedapproximately centrally on the substrate carries an active layer 2 andan n type first cladding layer 3. Current blocking layers are producedat each side of the mesa portion 1'. In the illustrated embodiment, thecurrent blocking structure include a p type embedded layer 4, an n typecurrent blocking layer 5, and a p type current blocking layer 6successively grown on either side of the mesa portion 1'. An n typecladding layer 7 covers the entire upper surface of the semiconductordevice including the n cladding layer 3 and p type current blockinglayer 6. Laser devices of this type are often characterized as buriedheterostructure devices.

It is important to note in the device illustrated in FIG. 1, that the ntype current blocking layer 5 terminates before and thus does notcontact the mesa portion and is thus electrically isolated from the ncladding layers 3, 7. Thus, although there is the desired reverse biasedp-n junction between the n type current blocking layer 5 and itsadjacent p type current blocking layer 6, there is no contact betweenthe n type current blocking layer 5 and either of the n cladding layers3, 7 and thus no leakage path for current which might flow through thep-n junction. As a result of the elimination of the contact andresulting leakage, the semiconductor laser device illustrated in FIG. 1operates with greater efficiency than prior art devices such as thatillustrated in FIG. 6.

Those skilled in the art will appreciate that contacts and the likelater added to the semiconductor laser device have not been shown forclarity of illustration of the invention. Those skilled in the art willalso appreciate that the laser device is typically of the GaAs typeformed by liquid phase epitaxial growth in which InP is usually used asthe base for the melt with Zn impurities added when p type layer isdesired and Te impurities added when an n type layer is desired.

Turning now to FIGS. 2a-2c, the process for producing the semiconductorlaser device of FIG. 1 will be described in greater detail. Thepartially completed device shown in FIG. 2a is that which results from afirst stage liquid phase epitaxial growth process. In that first stageprocess, the semiconductor substrate is produced and the active layer 2and the first n cladding layer 3 are successively grown on thatsubstrate. Having completed the first stage growth, the mesa portion1'is produced using conventional techniques. Such techniques typicallyinclude a photolithographic process to apply a mask defining the surfaceof the mesa portion, deposition of an etch resistant such as Si0₂ overthe surface of the mesa portion, and an etching process to removeportions of the n cladding layer 3, active layer 2 and substrate 1 oneither side of the mesa to produce the partially completed deviceillustrated in FIG. 2b.

Following the formation of the mesa portion, the partially completedsemiconductor is then subjected to a second stage liquid phase epitaxialgrowth which is either followed by or combined with a diffusion processwhich eliminates contact between the n current blocking layer 5 and then cladding layers 3, 7. During the first portion of the second stageepitaxial growth process, p type embedded layers 4 are grown on eitherside of the mesa portion. Additional layers are grown on either side ofthe mesa portion including the n type current blocking layer 5 and ptype current blocking layer 6. Finally, an n type cladding layer 7 isgrown over the upper surface of the partially completed semiconductordevice to cover both the p type current blocking layer 6 and n typecladding layer 3. It will be appreciated that the liquid phase epitaxialgrowth process causes the layers 4-6 grown at either side of the mesaportion to grow to points 6' which terminate at the very edge of themesa portion. Thus, the n current blocking layer 5 will, as in the priorart, tend to contact one or the other of the n cladding layers 3, 7 andthus provide a path for leakage.

In accordance with the invention, however, a tip portion 8 of the n typecurrent blocking layers 5 is inverted to the opposite conductivity typeto eliminate the electrical connection and any leakage path resultingtherefrom. As will be described in greater detail below, impurities inthe p or n layers are diffused by thermal diffusion to invert theconductivity type of tip 8 of the current blocking layer 5. In theillustrated embodiment, the inversion is from n type to p type such thatthe tip portion 8 represents a p type separation preventing connectionbetween the n type material of the current blocking layer 5 and the ncladding layers 3, 7. In the following, the process of achieving theinversion by thermal diffusion will be described in connection with twospecific examples. It should be appreciated, however, that there arenumerous process variations which can be incorporated without departingfrom the scope of the claimed invention.

As a first aspect of the conductivity inversion process, the impurityconcentrations for the n and p type layers (typically Te and Zn,respectively) are adjusted with respect to each other so that thermaldiffusion within a practical time limit can achieve the desiredinversion. In the prior art, the concentrations used are such that the ntype impurity concentration is typically 5 to 10 or more times that ofthe p type impurity concentration.

As a first example of practicing the invention, the impurity levels areadjusted for the second stage epitaxial growth process such that the ptype impurity concentration for the layers 4, 6 is greater than the ntype impurity concentration for the layer 5. Elevated diffusiontemperatures applied to the device cause the p type impurities in thelayers 4, 6 to diffuse into the layer 5 as suggested by the dashed linesassociated with layer 5 in FIG. 2. The diffusion time is adjusted suchthat diffusion occurs throughout the thin tip portion 8 of the layer 5but is insubstantial as compared to the thickness of the main body ofthe layer 5. Thus, the tip portion 8 is inverted from n type to p typeconductivity and the desired separation between the n type layer 5 andthe n type cladding layers 3, 7 is achieved.

Process conditions for accomplishing the inversion just described areillustrated in FIG. 4. In FIG. 4, the horizontal axis representsdiffusion time and the vertical axis impurity concentration for both then and p type impurities. The variable n₁ represents the initial n typeimpurity concentration in the n type current blocking layer 5. The word"initial" is used to refer to impurity concentration at the commencementof the diffusion process and thus relates directly to the impurityconcentration in the melt. Similarly, p₂ and p₃ represent the initialvalues of the p type concentrations in the p type embedded layer 4 and ptype current blocking layer 6, respectively. FIG. 4 illustrates thesituation in which there is no p type impurity in the n type currentblocking layer 5 at the start of diffusion. The plot shows the changewith respect to time of impurity concentrations in the n type currentblocking layer 5 at the points A and B (see FIG. 2), i.e., at the tipportion 8 and in the body, respectively, of the layer 5. Thus, forexample, n_(A) represents the n type impurity concentration at point Awhile p_(B) represents the p type impurity concentration at point B, andtheir change with respect to diffusion time.

Since the diffusion speed of the n type impurities (typically Te) isquite low with respect to p type impurities (typically Zn), for the timeperiod considered in FIG. 4, there is little change in the n typeconcentration at the points A and B, and it remains very near itsinitial value n₁. On the other hand, because of the higher diffusionspeed of the p type impurities, and because of the relatively highconcentration of p type impurities in the layers 4, 6, there is asignificant change in the p type concentration at both points A and B.Indeed, if diffusion is conducted for a sufficient length of time, theimpurity levels at both points A and B can change from substantiallyzero to substantially p₂, p₃. It will be apparent that the conductivitytypes at the points A and B are n types when the n concentration isgreater than the p concentration, and p types when n is less than p.Therefore, in the time range between t₁ and t₂, it is possible to invertthe conductivity type at point A, i.e., in the tip portion 8 of thelayer 5, while not inverting the conductivity type at the point B in thebody portion. Thus, the tip portion conductivity is changed to producethe desired separation, while the body portion remains n type to serveas a current blocking layer. As exemplary impurity concentrations, n₁=5×10¹⁸ cm⁻³, p₂, p₃ =5.5× 10¹⁸ cm⁻³. A diffusion temperature of 600° C.and time of approximately 10 minutes will produce the necessaryinversion in the tip portion 8 while retaining the n type conductivityof the body portion.

Utilizing the aforementioned parameters, it is possible to conduct thediffusion in a relatively short time because the inversion is determinedlargely by the p type impurities which have a relatively high diffusionspeed. However, this approach has a disadvantage in that the control ofthe size of the diffusion region can be difficult.

A second set of process parameters can be utilized which lengthen thediffusion time but also provide additional controllability. In anexample of such a process, the impurity concentrations are adjusted suchthat n₁ is somewhat greater than p₂ and p₃. FIG. 5 illustrates theinitial conditions as well as the changes in impurity concentration atpoints A and B with diffusion time. It will be appreciated, however,that the time axis of FIG. 5 is not on the same scale as FIG. 4 andindeed the plot of FIG. 5 represents a much greater total diffusion timethan that of FIG. 4.

As can be seen from FIG. 5, the conductivity type inversion isdetermined by the diffusion of n type impurities. Early in the diffusioncycle, the impurity concentration at both points A and B had increasedfrom about zero to substantially p₂, p₃, the initial impurityconcentration levels of the p type layers 4, 6. However, since thatinitial concentration is lower than the n type concentration, noconductivity inversion had occurred at that point. However, at the timet₁, sufficient n type impurities are diffused out of the n type layer 5to invert the conductivity type at the point A, i.e., in the tip 8 ofthe layer 5. At time t₁, however, the impurity level n_(B) at the pointB remains greater than the p type level p_(B) at that point, and noconductivity inversion has occurred at point B. Thus, as in the priorexample, the inversion has occurred between the times t₁ and t₂ and theprocess conditions are adjusted to achieve a conductivity inversion atpoint A but avoid such an inversion at point B. Exemplary impurityconcentrations in connection with this example are n₁ =5×10¹⁸ cm⁻³ andp₂, p₃ =2×10¹⁸ cm⁻³. In order to achieve the desired inversion, the 600°C. diffusion temperature can be applied for approximately one hour.

It will now be apparent that other combinations of process conditions,including impurity concentration levels, their adjustment with respectto each other, diffusion temperatures and times can be selected toachieve the partial conductivity inversion taught herein. For example,in the case of FIG. 4, where p₂ and p₃ are greater than n₁, if the ptype concentration is made substantially greater than the n type thediffusion time is quite short. However, as the p type concentrations arereduced with respect to the n type, the diffusion time increases.Similarly, in the case of FIG. 5 where the initial n type concentrationis greater than the initial p type concentration, if n₁ is made muchgreater than p₂ and p₃, the diffusion time to achieve an inversion isquite long. However, as n₁ approaches p₂ and p₃, the diffusion time toachieve inversion substantially reduces.

In both the examples of FIGS. 4 and 5, if the diffusion temperature isincreased, the diffusion proceeds more quickly and the diffusion timemust be reduced in order to achieve the desired partial inversion. Thus,the values n₁, p₂ and p₃ are related and can be adjusted along withdiffusion time and diffusion temperature to achieve the desired result.

In addition, it is possible to utilize other diffusion conditions if thetip portion 8 of the layer 5 is made thinner by altering the conditionsof the crystal growth.

In the foregoing examples, the initial p type impurity concentrations p₂and p₃ were considered to be the same for purposes of convenience.However, in some circumstances, it may be desirable to utilize differentp type concentrations for the layers 4 and 6 and adjust diffusionconditions appropriately.

It will also be apparent that the diffusion process can be combined withor separated from the second stage crystal growth process, as desired.More particularly, diffusion may be conducted as a completely separateprocess after crystal growth is completed. Alternatively, the diffusioncan be conducted as a part of the crystal growth process and immediatelyafter growing of the crystal layers by holding the grown crystal at ahigh temperature for the required time. The crystal need not be removedfrom the crystal growing apparatus for the diffusion since no diffusionmask is required.

In the embodiments described above, the mesa configuration is a forwardmesa configuration, i.e., one in which the mesa base is larger than thetop portion of the mesa. It will be apparent that conventionaltechniques can be used to produce a reverse mesa configuration, and theinvention can be applied to the reverse mesa configuration asillustrated in FIG. 3. In FIG. 3, as in FIG. 2, the p type substrate 1,active layer 2 and n type cladding layer 3 are formed in a first phasecrystal growth process. The mesa is then etched and p type embeddedlayer 4, n type current blocking layer 5 and p type current blockinglayer 6 are grown at either side of the mesa followed by an n typecladding layer 7 covering the entire upper surface of the device. As inthe earlier cases, the n type layer 5, when initially grown, came to apoint at the corner of the mesa, but a conductivity inversion by thermaldiffusion was accomplished to isolate the n type current blocking layer5 from the n cladding layers 3, 7, thereby to prevent leakage.

It will now be apparent that what has been provided is a low thresholdcurrent, high efficiency semiconductor laser device which has a low idlecurrent by virtue of the minimization of current leakage through thedevice. The method for producing the device includes a thermal diffusionprocess which partly inverts the conductivity type of one of the layersthereby eliminating what had been a troublesome leakage path in theprior art.

What is claimed is:
 1. A method of producing a semiconductor laserdevice comprising:growing an active layer and a second conductivity typefirst cladding layer successively on a first conductivity typesemiconductor substrate; etching the active and first cladding layers oneither side of a central stripe to produce a mesa-portion; growingadditional layers having predetermined concentrations of impurities onthe semiconductor substrate on either side of the mesa portion, thesuccessively grown layers including a first conductivity type embeddedlayer, a second conductivity type first current blocking layer, and afirst conductivity type second current blocking layer; growing a secondconductivity type second cladding layer on the second current blockinglayer and the first cladding layer; and diffusing impurities into thefirst current blocking layer from at least one of the embedded layer andthe second current blocking layer for a time sufficient to invert theconductivity type of a tip portion of the first current blocking layerthereby to minimize leakage through the semiconductor laser device. 2.The method of producing a semiconductor laser device as set forth inclaim 1 including adjusting the impurity concentration levels such thatat least one of the embedded layer and the second current blocking layerhas a high impurity concentration relative to the first current blockinglayer, and wherein impurities diffused from the high impurityconcentration layer control the inversion of the conductivity type ofthe tip portion.
 3. The method of producing a semiconductor laser deviceas set forth in claim 1 including adjusting the impurity concentrationlevels such that the first current blocking layer has a high impurityconcentration relative to the embedded layer and the second currentblocking layer, and wherein impurities diffused from the first currentblocking layer control the inversion of the conductivity type of the tipportion.
 4. The method of producing a semiconductor laser device asdefined in claim 1 wherein said diffusion is conducted during theprocess of growing the layers on the semiconductor substrate.